1. Field of the Invention
The present invention relates to a novel Mm--Ni type hydrogen storage alloy for Ni/MH (metal hydride) secondary cells. More particularly, the present invention relates to an Mm--Ni type hydrogen storage alloy which is of low production cost with high performance and high discharge capacity.
2. Description of the Prior Art
Hydrogen storage alloys are the metals or alloys which are able to absorb or discharge hydrogen reversibly at certain temperatures under certain pressures. In order for the hydrogen storage alloys to be applied in practice, they are required to show large hydrogen storage capacities which are reversibly available as well as to show long electrode life spans in electrolytes.
The hydrogen storage alloys for Ni/MH secondary cell, developed thus far, can be exemplified largely by two types: AB.sub.5 type including Mm--Ni, wherein A is an element having a high affinity for hydrogen, that is, a rare-earth element, such as La, Ce, Pr, Nd, etc, and B is a transition metal qt transition metals selected from Ni, Mn, Co, Fe, Al, etc; and AB.sub.2 type including Zr--Ni and Ti--Ni. The former AB.sub.5 type is disadvantageous in that its energy storage density is low while the latter AB.sub.2 type is poor in almost all of its functions. Thus, in order to develop the Ni/NH secondary cells which are of high capacity and high performance, it is necessary to research for the high capacity and the long electrode life span of AB.sub.5 type hydrogen storage alloys for which higher performances are secured than for AB.sub.2 type hydrogen storage alloys.
Recently, extensive research has been directed to the development of the anode materials for Ni/MH secondary cells. In most case, the research was focused on AB.sub.5 type hydrogen storage alloys and resulted in MmNi.sub.5 type alloys with an electrochemical discharge capacity of about 200-300 mAh/g.
However, the miniaturization of electronic equipment requires alloys which are of higher discharge capacity and better electrode life span than the conventional MmNi.sub.5 type alloys. This requirement is also raised by the development of electric vehicles which demand high capacity and high performance batteries.
In addition, the development of other types of high-grade cells, together with the expansion of the secondary cell market, compels the production cost of conventional Ni/MH to be reduced.
It is reported in J. Less-Common. Met., 129 (19897) 1 by J. J. Wilem et al. that, when Co is used instead of the Ni of La--Nd--Ni type hydrogen storage alloy electrodes, the resulting alloy electrodes have long life spans because the cobalt-substituted alloys are lowered in pulverization rate.
A similar report is disclosed in J. Less-Common Metal., 172-174 (1991) 1175 by T. Sakai et al. in which the improvement in the life spans of Mm--Ni--Al--Mn hydrogen storage alloy electrodes can be also achieved through the replacement of Co for Ni.
However, the supra alloys suggested are more expensive than the conventional metal alloys. Further, the major element Co shows a low affinity for hydrogen, resulting in high production cost and low discharge capacity.
T. Sakai et al., continued to conduct research for improvement in the life spans of AB.sub.5 type hydrogen storage alloy electrodes without deteriorating their discharge capacities and reported in J. Electrochemical Society, Vol. 134, No. 3, (1987) 558 that the life spans of AB.sub.5 type hydrogen storage alloy electrode could be extended by electroless plating Cu and Ni on alloy powders. The electroless plating processes suggested by T. Sakai et al., however, are difficult to put into practice because the additional processes produce pollution of the environment as well as increase the production cost. What is worse, the improvement in the life span is less effected by the non-electrolyte plating than by the Co replacement.
As mentioned above, the life span of electrodes in electrolyte, which has been one of the hottest issues in AB.sub.5 type hydrogen storage alloy art, can be extended by a decrease in the pulverization rate of the alloy, which is accomplished by the substitution of the Co element to the alloy.
The composition of a commercially available Mm type hydrogen storage alloy is given as shown in Table 1, below.
TABLE 1 ______________________________________ Elements in a conventional Mishi metal-based Hydrogen Storage Alloy Unit Price Content Cost Share Elements ($/lb) (wt %) (%) ______________________________________ Mm 4.0 33 23 Ni 3.5 50 30 Co 28.0 20 47 Mn 0.7 5 &lt;1 Al 0.6 2 &lt;1 ______________________________________
The Mm type hydrogen storage alloy of Table 1 comprises 10% by weight of Co and can be represented by MmNi.sub.3.55 Co.sub.0.75 Mn.sub.0.4 Al.sub.0.3. This commercially available alloy can give the electrode an extended life span by virtue of its large Co fraction but gives rise to an increase in production cost: the Co element amounts to 47% in cost share.